On my PC I have software that lets me easily reduce image sizes and join together multiple pics into one image. So far, I haven’t found a good app to do this one my phone. Furthermore, sometimes the apps are ad-supported and the ads take up too much screen space.

I used Image Size with iPhone, I find that they have it with Android. You can change the resolution size and crop the photo. Can’t say it is the best app, but it is okay to do what I want (and without paying).